1. Field of the Invention
The invention relates to a new liquid crystal compound useful as a material for a display device, and a liquid crystal composition. The invention relates more specifically to a new liquid crystal compound having a wide temperature range of liquid crystal phases, a high clearing point, a large dielectric anisotropy, a large refractive index anisotropy, and a good compatibility with other liquid crystal compounds, and also being usable in a wide temperature range, drivable at a low voltage, and possible to exhibit a steep electrooptical characteristics when used for a liquid crystal display device, and to the liquid crystal display device comprising the composition.
2. Description of Related Art
Display devices using liquid crystal compounds (The term liquid crystal compound is used in this specification as a generic term for a compound having a liquid crystal phase and a compound having no liquid crystal phases but useful as a component of a liquid crystal composition.) have been widely applied to displays for timepieces, electronic calculators, word processors, and so forth. In these display devices, the refractive index anisotropy, the dielectric anisotropy, and so forth of the liquid crystal compounds have been utilized.
In the liquid crystal display devices, a classification based on the operation mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. A classification based on the driving mode of devices includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth.
These liquid crystal display devices comprise liquid crystal compositions having suitable physical properties. The liquid crystal compositions desirably have suitable physical properties for improving the characteristics of the devices. General physical properties necessary for liquid crystal compounds which are the components of the liquid crystal compositions are as follows:
(1) being chemically stable and physically stable,
(2) having a high clearing point (the phase transition temperature of a liquid crystal phase to an isotropic phase),
(3) being low in the minimum temperature of liquid crystal phases (a nematic phase, a smectic phase and so forth), especially of the nematic phase,
(4) having an excellent compatibility with other liquid crystal compounds,
(5) having an appropriate dielectric anisotropy, and
(6) having an appropriate optical anisotropy.
A voltage holding ratio can be increased by using a composition containing a chemically and physically stable liquid crystal compound as described in item (1).
The temperature range of a nematic phase can be widened in a composition containing a liquid crystal compound having a high clearing point or a low minimum temperature of liquid crystal phases as described in items (2) and (3), and thus a display device is usable in a wide temperature range.
The liquid crystal compound is generally used as a composition prepared by being mixed with many other liquid crystal compounds to exhibit characteristics which cannot be attained with a single compound. Thus, it is desirable that a liquid crystal compound used for a display device has a good compatibility with other liquid crystal compounds and so forth, as described in item (4).
Recently, a liquid crystal display device has been required to have higher quality especially in display performance such as contrast, display capacity, and response time. Further, a liquid crystal material to be used has been required to have a lower driving voltage, that is, a liquid crystal compound capable of decreasing threshold voltage and a liquid crystal composition with a lower driving voltage comprising this compound has been required.
The threshold voltage (Vth), as is well known, is represented by the following equation. Refer to H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81.Vth=π(K/∈0Δ∈)1/2 In the equation above, K is an elastic constant and ∈0 is a dielectric constant in a vacuum. As is shown by the equation, two methods are possible to decrease the Vth, either by increasing the value of Δ∈ (dielectric anisotropy) or by decreasing K. However, K is not easily controlled by the present technology, and the present demand has been commonly dealt with the use of a liquid crystal material having a large Δ∈. Under these circumstances, liquid crystal compounds having an appropriate dielectric anisotropy as described in item (5), and in particular, liquid crystal compounds having a large dielectric anisotropy have been developed actively.
It is desirable that the cell thickness of a liquid crystal display device and the value of refractive index anisotropy (Δn) of a liquid crystal material used are constant to attain a good liquid crystal display. Refer to E. Jakeman, et al., Phys. Lett., 39A. 69 (1972). The response speed of a liquid crystal display device is in inverse proportion to the square of the thickness of a cell used. Thus, a liquid crystal composition having a large refractive index anisotropy should be used to manufacture a liquid crystal display device capable of a high-speed response and applicable to the display of moving images and so forth. Accordingly, a liquid crystal compound having an appropriate refractive index anisotropy as described in item (6), and a liquid crystal compound having an especially large refractive index anisotropy have been demanded.
A variety of liquid crystal compounds having a large dielectric anisotropy and refractive index anisotropy have been synthesized until now, and some of them are used practically. For example, four-ring compounds having a CF2O bonding group are disclosed in patent documents 1 to 6. However, these compounds do not have an sufficiently high clearing point, so that the usable temperature range of their compositions are not sufficiently wide for the use of display devices.
Further, five-ring compounds (compounds (S-1) to (S-3)) comprising a tetrahydropyran ring and having a CF2O bonding group are disclosed in patent documents 7 to 12. None of these compounds also have a sufficiently high clearing point. Compounds (compounds (S-4) to (S-5)) comprising a tetrahydropyran ring and a dioxane ring are also disclosed in patent document 11. However, these compounds do not have a sufficiently large refractive index anisotropy.

The patent documents cited are No. 1: WO 1996/11897 A;                No. 2: JP H10-204016 A (1998);        No. 3: GB 2229438 B;        No. 4; DE 4023106 A;        No. 5; JP H10-251186 A (1998);        No. 6; WO 2004/035710 A;        No. 7; WO 2004/048501 A;        No. 8; JP 2004-352721 A;        No. 9; WO 2005/019378 A;        No. 10; WO 2005/019381 A;        No. 11; WO 2006/125511 A; and        No. 12; WO 2006/125530 A.        